In the AVM cell, AHR-1 elevates MEC-3 expression as well

as blocks downstream Paclitaxel in vivo MEC-3 targets that result in traits normally reserved for PVD (e.g., lateral branching, sensitivity to low temperatures). Thus, AHR-1 is required for the twinned tasks of inducing the light touch fate while simultaneously preventing expression of nociceptor genes. We show that one of these targets, the claudin-like membrane protein HPO-30, acts in PVD to stabilize lateral dendrites. We hypothesize that HPO-30/claudin maintains PVD dendritic branches by mediating adhesive interactions with the adjacent epidermis. HPO-30 is ectopically expressed in the ahr-1 mutant AVM cell and is required for its PVD-like morphology. We note that this effect is remarkably similar to that of the mutant phenotype for the Drosophila AHR-1 homolog, Spineless,

in which simple sensory neurons adopt more complex arbors, although the Spineless targets that effect this outcome are not known ( Kim et al., 2006). The strong conservation of this role in dendritic branching suggests that the vertebrate Spineless homolog is likely to exercise a similar function, and thus that the downstream effector molecules that we have identified in C. elegans may also pattern the architecture of mammalian sensory neurons. C. elegans responds to physical stimuli through a diverse array of mechanosensory neurons ( Chatzigeorgiou et al., 2010b, Geffeney SB431542 et al., 2011, Chalfie and Sulston, 1981 and Hall and Treinin, 2011). Light touch

to the body (posterior to pharynx) is mediated by six TRNs (AVM, PVM, PLML, PLMR, ALMR, and ALML), whereas a harsh mechanical stimulus to this region is detected by PVDL and PVDR ( Figure 1) ( Way and Chalfie, 1989). These neurons occupy unique locations and adopt distinct branching patterns. The touch receptor neurons display a simple morphology with unbranched longitudinal processes emanating from the cell soma. In contrast, the “harsh-touch” PVD MRIP neurons are highly branched with elaborate dendritic arbors that envelop the animal in a net-like array ( Figure 1) ( Halevi et al., 2002, Oren-Suissa et al., 2010, Smith et al., 2010 and Tsalik et al., 2003). FLP neurons in the head, which also respond to harsh mechanical force ( Chatzigeorgiou and Schafer, 2011), show a similar PVD-like pattern of orthogonal dendritic branches ( Albeg et al., 2011 and Smith et al., 2010). PVD displays additional sensory responses to temperature and hyperosmolarity ( Chatzigeorgiou et al., 2010b) (shown later in Figure 4). The members of these subgroups of mechanosensory neurons are also distinguished by their developmental origins. The touch neurons ALMR, ALML, PLMR, and PLML are generated in the embryo ( Sulston et al., 1983). AVM and PVM are each produced during the first larval (L1) stage by unique patterns of cell migration and division of Q-cell progenitors on the left (PVM) and right (AVM) sides of the body ( Sulston and Horvitz, 1977).